Field of the Invention
The present invention relates to an image processing apparatus that is capable of properly emphasizing differences in brightness between bright spots, an image processing method, and a storage medium, and more particularly to a technique that is applied to photographing of the starry sky.
Description of the Related Art
In recent years, a technique of reducing noise in a digital camera has been improved, and this makes it possible to obtain a high-quality image also in high-sensitivity photographing. As a consequence, digital cameras are equipped with a function which makes it possible to easily perform celestial photographing which has been conventionally difficult to be performed even in a state where a digital camera is fixed.
In the case of photographing the starry sky, in a suburb, a mountain area or the like, where there are few lights in the surrounding area, it is possible to photograph stars ranging from bright stars to low-brightness stars, such as faint stars, because the sky is dark. However, in a city area or a residential area, the sky is bright due to e.g. an influence of light pollution, and hence it is difficult to perform celestial photographing such that a large number of stars are included in a photographed image. To meet a demand for photographing a large number of stars even in the night sky under the influence of light pollution, there has been proposed a method of emphasizing a low-brightness star by performing image processing for emphasizing edges of a spot which is higher in luminance than its surrounding area. As such star emphasizing processing, there has been proposed a method of automatically determining a weighting amount of a threshold value for use in binarization for identifying a point light source area from an image area, and detecting bright spots included in the image with high accuracy by using the determined weighting amount (see Japanese Patent Laid-Open Publication No. 2013-25650).
According to the above-mentioned star emphasis processing, when photographing the night sky in a bright place under the influence of light pollution or the like, it is possible to emphasize a low-brightness star by performing edge emphasis for pixels higher in luminance than its surrounding area. However, when photographing the night sky in a dark place, a photographed image becomes an unnatural one because all stars are emphasized, and further it is impossible to show bright stars in an emphasized manner out of a large number of stars in the photographed image.